The present invention relates to orifice assemblies for fluid flow conduits and more particularly to orifice assemblies for increasing the resistance to fluid flowing through such conduits. The present invention has particular application in forced circulation steam generating units, but may be applied in any hydraulic system.
In steam generating units constructed to operate with forced circulation, a multiplicity of steam generating tubes are connected to a common header from which they receive their supply of water thereby forming a multiplicity of parallel tube circuits in which steam is generated. The common header may be connected to the discharge of the pump which receives its water from a steam and water separating drum. The tube circuits discharge the steam and water mixture into the steam and water separating drum.
In the past, orifices or other flow restricting means have been employed at the entrance of the multiplicity of tubes for controlling the distribution of water from the common header to the individual tube circuits. A high flow resistance is required to assure uniform flow distribution and prevention of flow reversals in shaded waterwall panels and flow starvation of adjacent tubes in the event of a single tube rupture. In connection with the latter aspect of prevention of flow starvation, the orifices serve to throttle or choke the flow to the ruptured steam generating tube. This then insures that the remaining mass flow from the common header will be distributed to the remaining, nonruptured, steam generating tubes.
As is well known in the art, choking of flow through an orifice is dependent on the pressure drop across the orifice which in turn is dependent on the fluid resistance presented by the orifice and the square of the flow rate through the orifice. For example, with a relatively high resistance presented by the orifice, the phenomenon of choking will occur at a lower mass flow rate than would occur with a lower resistance. Accordingly, it is highly desirable to achieve the greatest possible flow resistance of orifices so that should a steam generator tube rupture, a greater mass flow rate will be available for cooling of the remaining steam generator tubes served by the common header.
Set off against the desirability of increasing the flow resistance offered by an orifice is the fact that very small openings have a tendency to foul which itself has a very adverse effect upon operation of the steam generating unit. Accordingly, the problem presents itself as how to obtain a greater flow resistance than that obtainable with an orifice having the smallest opening conducive to anti-fouling operation.
One solution which has been suggested involves stacking of several orifices, each having the minimum opening allowed to prevent adverse effects of fouling. However, this has not proved workable because the flow issuing from an orifice requires a substantial distance downstream before it fully expands back to the constraints of a tube and as such requires a substantial spacing between adjacent orifices in order to take full advantage of the flow resistant characteristics of the orifices. This substantial spacing is not acceptable in steam generating units where it is desired to choke flow at the inlet to the steam generating tubes.